This invention relates generally to a laser projection system, and more specifically to a laser imaging system incorporating speckle suppression.
Recently, projection systems have been created that utilize lasers as a light source. Two primary forms of laser projection systems exist. One laser projection system uses a laser and a raster technique to write an image pixel by pixel to a projection surface. Another laser projection system uses a laser as an illumination source and a spatial light modulator, such as a LCD, to project an image in its entirety onto a projection surface.
Laser projection systems using a raster procedure operate by deflecting a beam of coherent light generated by a laser to form an image. The deflectors include devices such as spinning mirrors and acousto-optic deflectors (AODs). Red, green, and blue light from laser sources can be independently modulated, and then combined and scanned onto a surface using a polygon mirror or galvanometer in a color projection system.
Unfortunately, there are a number of problems associated with laser raster projection systems. For example, flicker places an upper limit on the number of pixels displayable. Only one pixel can be displayed at any given moment due to the nature of the deflectors. Furthermore, there is no persistence to the display. Since laser projection systems direct the light onto a diffusion surface, all points to be displayed must be illuminated within a time period less than the critical flicker frequency (CFF) of the human eye.
Another problem with laser raster projection systems relates to the generation of color images, which requires the use of multi-colored lasers. Substantial difficulties exist in aligning and synchronizing multiple deflectors so as to simultaneously image different colors at a given pixel location.
Liquid Crystal Displays (LCDs) are also used in projection systems that may produce speckled images. Light sources used for LCD projection systems include incandescent lamps, arc lamps, Light Emitting Diodes (LEDs) and lasers. While LEDs emit incoherent light that would not produce speckle patterns in a projected image, they do not output sufficient light for projection systems. Lamps are brighter than LEDs, but not bright enough to be used in projection systems for large screens and lamps generate considerable heat. Lasers can be used as a light source for a LCD projection system since they are capable of outputting more usable light, thus providing a very bright image over a large area.
Lasers used as light sources for laser raster or LCD projection systems produce an undesirable speckle pattern in a projected image. Laser speckle is an interference pattern that results from the reflection or transmission of highly coherent light from an optically rough surface, one whose local irregularities in depth are greater than one quarter of a wavelength. For example, if a laser beam is directed at a wall, a bright spot with a surrounding distribution of speckles is observed, rather than a uniformly illuminated spot. Laser light directed at an uneven projection surface is reflected as different phases of light. The human eye perceives these different phases as interference. Thus, the mutual interference of partially coherent beams causes the speckle pattern.
Various systems and methods have been attempted in the prior art to address speckle elimination or suppression in a projected image. Speckle reduction techniques relying on mechanical motion can be difficult to implement and are prone to failure. Diffusers are also capable of reducing speckle, but tend to be extremely loose. U.S. Pat. No. 4,011,403 discloses an object-illuminating and imaging system comprised of a laser as a light source and an optical fiber as a light transmitter. A light-flow-disruptive means acts upon the collimated illumination to reduce objectionable speckle effects. In one embodiment, a diffuser such as polytetrafluoroethylene lens or disc may be interposed in the light flow path at a location between the light source and the object viewed to reduce objectionable speckle. Alternatively, speckle effect may be reduced by vibrating one of the elements in the optical path traversed by the light beam. A speckle reduction system that uses a diffusing element, however, will result in a display system with significant optical power losses.
U.S. Pat. No. 4,155,630 discloses a process and apparatus for improving image quality by speckle elimination in a coherent light imaging system. Diffused laser light is directed onto a mirror having a rocking motion that will cause the reflected rays to sweep a two-dimensional area. The reflected light is focused through a diffuser before collimating the light for use in image creation. However, the use of both a rocking mirror and a diffuser for speckle elimination in an imaging system results in significant losses of optical power.
U.S. Pat. No. 5,272,473 discloses a coherent light projection system and method having reduced speckle. The invention is comprised of a coherent light source and a display screen. The coherent light source generates a plurality of narrow light beams that impinge on the display screen at a plurality of associated points. The display screen has a transducer that generates surface acoustic waves that traverse the associated points. Significant movement of the display screen such as to sufficiently reduce noticeable speckle is difficult, especially when the display screen is large.
U.S. Pat. No. 5,274,494 discloses a method and apparatus for reducing or eliminating the speckle intensity of distribution in a highly coherent source. A coherent beam of light is directed into Raman cells to obtain a broad spectral bandwidth beam of light having additional side wavelengths other than the original wavelength. The composite beam having broad spectral bandwidth is capable of forming images that are substantially free of speckle intensity source. It is necessary to compensate for spatial incoherence that the Raman cells cause. Furthermore, the use of multiple Raman cells introduces significant complexity in a speckle reduction system.
U.S. Pat. No. 5,313,479 discloses a system and method for a speckle-free display system comprised of at least one coherent light source, a diffusing element located in a plane intercepts coherent light beam, a spatial light modulator for receiving the diffused light beam and for generating an image light beam, and a viewing surface. The movement of the diffusing element causes the speckle interference pattern to move on the viewing surface. The diffusing element can be rotated or vibrated. A speckle reduction system that uses a rotating diffusing element, however, will result in a display system with significant optical power losses.
In addition to image projection systems incorporating speckle reduction, lithographic systems have utilized speckle reduction techniques. U.S. Pat. No. 5,453,814 discloses a uniform illumination system and method for microlithography. A light source, having a solid state laser, emits a beam that is separated into a number of segments. Segments are frequency shifted by a different amount such that they do not substantially overlap in the frequency domain. Each segment passes through a short focal length lens element of a fly""s eye array to be dispersed onto a mask plane for uniformly illuminating a mask. The lens element of the fly""s eye array are contained within a small region in comparison to the width of the dispersed beam segments, such that each beam segment contributes illumination to the entirety of a common portion of the mask. The system provides uniform illumination in the deep ultra-violet range without speckles or fringes. The fly""s eye array of frequency shifting elements, each element of which shifts by a different frequency, is an optically complex means of incorporating speckle reduction in an image projection system.
It is, therefore, desirable to reduce speckle in a laser projection system by non-mechanical or diffusion means.
It is an object of the present invention to provide a system and method for speckle suppression in laser projection systems.
The present invention is directed to overcome the problem set forth above. Briefly summarized according to one aspect of the present invention, a radio frequency (RF) signal is injected into at least one semiconductor laser that acts as the light source for a projection system. The injection of radio frequency changes the laser emission mode structure rapidly, thus producing multiple longitudinal lasing modes, each of which exhibits a different speckle pattern. Images projected in a projection system comprises of radio frequency injected lasers will suppress the appearance of unwanted speckle, since the differing speckle patterns produced by the different laser operational modes will be superimposed and will blend together on a projection surface.
In a further aspect of the invention, the wavelength of a laser light source for a projection system is Doppler shifted to produce different speckle patterns. In one embodiment, the laser projection system is comprised of at least one laser and an acousto-optic modulator (AOM) which changes the laser wavelength by the RF carrier frequency to the AOM. Slewing the RF carrier frequency Doppler shifts the laser wavelength, alters the speckle pattern, and reduces the delectability of the speckle in the image formed on a projection surface.
In an additional aspect of the invention, a deflector directionally moves the output beam of a laser projection system to reduce noticeable speckle in an image on a projection surface. The pointing angle of the deflected beam from the AOM is a strong function of the carrier frequency. By slewing the frequency, the beam location on the screen can be changed, thus changing the speckle pattern. The movement of the beam need only be on the order of 1 pixel which does not significantly affect the modulation transfer function (MTF) in the direction of the movement. Beam movement at approximately 45 degrees to the vertical or horizontal can minimize the loss of MTF in the horizontal and vertical directions, and be less perceptible to the eye.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.